Ink brand permanency on plastic and metallic IC packaging has always been a critical issue in the semiconductor industry. Lid brands often identify information about the device type, the IC manufacturer, and the date of production. It is not uncommon for semiconductor users to subject IC packages to various chemical cleans before and after attachment to PC boards. If ink adhesion is not satisfactory and/or ink cure is incomplete, the brand may be degraded during the various cleaning operations, resulting in partially or completely illegible brands.
Long-term exposure to reactive ambients (e.g. high humidity, corrosive or polluted air, etc.) can also cause slow degradation of poorly adhered or cured ink brands. Loss of the branding information can result in IC's which are unidentifiable. For this reason, MIL-STD tests have been developed and instituted in the semiconductor industry to evaluate ink brand permanency on package lids. Moisture resistance tests (and salt air exposure) can also help determine brand permanency under more aggressive conditions.
Various lid cleaning, inking, and curing methods have been developed and modified to enhance the brand permanency on plastic molded compound surfaces and metallic packages or lids. One of the most difficult factors to control is the surface chemistry and cleanliness of the package. This directly affects the bonding of the ink to the package. The branding ink will not bond satisfactorily to package or lid surfaces that contain, for the plastic packages, inordinate quantities of old release agents, silicon oils, waxes, and surface organics and, for metal packages, an inordinate amount of metallic oxides or hydrocarbons or other contaminants. These surface contaminants can accumulate as a result of the mold, the molding compound, thermal treatments during the lid seal process and burn-in. Normal workplace air exposure, as well as machine and human handling (e.g. fingerprints), can also contribute to surface contamination of the package. Overall, the most common cause of brand loss is due to lifting or delaminating ink caused by poor ink bonding to the package.
Currently, branding operations for plastic packages, as recommended by the ink vendors, involves exposing the plastic package to solvents in order to clean the molding compound surface and/or exposing the package surface to a hydrogen flame prior to the ink branding. The solvents attempt to remove any chemical contaminants and the hydrogen flame ashes or burns the plastic package surface.
In particular, the solvents that are used to clean the plastic package are typically N-methyl-2-pyrrolidone or various halogenated materials such as 1,1,1-trichloroethane or tetrachloro-ethylene (TCE). The use of these solvents is coming under much closer scutiny, and is in many cases banned. The plastic package is exposed to the solvents by either immersing the package in the solution or by wiping the solvent across the package or lid surface with a wipe.
The other prior art, hydrogen flaming the plastic package, ashes or burns the surface of the package which removes the mold release agents, oils, waxes and surface organics and the like from the surface. However, the hydrogen flame will not effectively remove all of the surface organo-silicon complexes. Controlling the flame, especially in smaller packages, can also be difficult. This thermal exposure can cause localized stressing of the molding compound, which could lead to molding compound cracking, or delamination of the molding compound at the leads and/or die surface. This process can also only be applied to packages one at a time. In addition, many older style branders cannot be effectively retrofitted to accommodate a hydrogen flame apparatus. Various other flames have been used to flame the surface such as acetylene or butane instead of hydrogen.
For metal packaging, current branding operations for metallic packaging, and more specifically nickel plated packaging such as electroplated or electroless plated nickel lids or gold plated nickel, involve the use of wet chemical cleans. These include basic solutions, which have pH values greater than 10 (e.g. Markem 535), and solvents like isopropyl alcohol (IPA), freon, and HCFC. These solutions are recommended as standard surface cleaning methods by the ink vendors and generally result in improved brand permanency compared to uncleaned lids. However, one disadvantage of these wet chemical cleans is the entire package must be immersed.
The Markem 535 solution, for example, contains monoethanolamine and is quite basic (pH&gt;11). Hence, it tends to attack or etch various other materials associated with the packages. The attack is especially notable on the soldered leads, the lid-to-seal ring weld area, and the various plated metallic codes on the packages. This particular brand pre-clean can cause a darkening of these surfaces due to an accelerated build-up of oxides and the formation of `phospho-oxides` on electroless Ni plated surfaces.
The same aspects of providing a contaminant free surface stated above for plastic and metal also applies to ceramic packaging. The ceramic package material is also etched. In most cases, these anomalies are cosmetic. However, many incidents of darkened leads and/or darkened weld areas are incorrectly assumed to be caused by latent corrosion or thermal stress, when in fact, they are caused by the brand pre-clean solution.
Lead solderability is also affected by the pre-branding cleans. The oxides and `phospho-oxides` are difficult to remove and can prevent good eutectic formation between the base metal and the solder, resulting in de-wetting and no-wetting areas of solder. Leads solder dipped prior to brand also often exhibit a darkening and pitting of the solder. Reduced solderability is observed in these units as well.
Another drawback of the wet cleaning method is the use of solutions which are costly, difficult to dispose of, and are detrimental to the environment. Maintaining and changing these chemical baths, as well as keeping accurate records of bath changes and chemical disposal, are tedious and time consuming. In addition, several of these solutions can be potentially harmful to workers over a period of extended exposure.